Intelligent electronic devices (IEDs) are devices that are used for protection, control and monitoring of a power system network. The IED receives data from sensors and power equipment, and can issue control commands, such as tripping commands for opening e.g. circuit breakers, contactors or contact switches if they detect voltage, current, or frequency anomalies, or can effectuate a close command for closing circuit breaker or raising/lowering voltage levels in order to maintain a desired level.
The IED executes specific application functions on a platform which comprises hardware and firmware. The hardware platform typically comprises an analog handling part, for example transformer modules or A/D conversion, and provides input presented to a main Central Processing Unit/Digital Signal Processor (CPU/DSP) for processing. The main CPU/DSP is where the application functions are executed in the run-time environment. Binary status data from the power system network is transferred via binary input modules to the CPU/DSP for processing and logical computation. The commands to the process, for example a process such as opening and closing of a circuit breaker, are performed via binary output modules. All input/output modules either of analog or Boolean type communicates with the main CPU/DSP via a communication backplane or an external field/process bus. In addition, the IED can support a local machine interface screen, communication ports and time synchronization ports.
Controlled switching, or point-of-wave switching, of the circuit breakers is highly desirable in the power system network. Transient disturbances in power systems may damage equipment of the power system, and such voltage and current transients may be introduced during normal switching operations, e.g. during opening/closing operations of the circuit breaker. By means of a controlled switching of the circuit breaker, the harmful transients can be highly reduced. One of the most common applications for controlled switching of circuit breakers is in Shunt Capacitor Banks (SCB), during which high magnitude and high frequency transients can occur. Other application examples comprise Reactor Banks and Power Transformer bays.
In the controlled switching, sensors such as voltage transformers are used for measuring voltages at both sides of the circuit breaker and/or a sensor such as current transformer is used for measuring the current through the circuit breaker. In algorithms used for determining when to close the circuit breaker, parameters such as voltage difference, frequency difference and phase difference between the both sides of the current breaker are used. In the case when to open the circuit breaker, the conditions and algorithm can be different from the ones used for closing the circuit breaker. Algorithms for calculating such synchronization are known.
The implementation of point-on-wave switching comprises well-known algorithms for determining the best point for switching. In some applications, but not necessarily for all kinds of applications, a zero-current crossing is optimal, providing the least amount of current for the current breaker to break, thus providing the safest possible breaking and further e.g. minimizing wear on contactors of the circuit breaker and other damage to the electric power system or equipment.
However, to actually effectuate the switching at the desired switching point can be difficult, even when suitable algorithms for calculating it are used. One difficulty is various types of time delays. An example of such time delay is the operating time of the circuit breaker, i.e. the time for the circuit breaker to actually close (time from energizing a closing coil until contacts close), which for example could be some milliseconds. The CPU/DSP of the IED can be arranged to compensate for this circuit breaker operating time as the operating times are generally well known.
Still, there may be further delays introduced in the IED. As another example, when the CPU/DSP of the IED transmits a trip signal to the output module, this communication is conveyed in the earlier mentioned communication backplane or external field/process bus. Delays may be introduced in this communication backplane or external field/process bus, and the output module thus outputs the trip signal later than intended and the circuit breaker is tripped at another point of time than the intended.
From the above, it is clear that there is a need for an improvement on this situation in this field of technology.